Characterisation of source rocks is important for evaluation of both conventional and unconventional reservoirs. Organic matter is deposited and preserved at the bottom of lakes, seas and deltas. As more material is deposited, the organic matter is buried and the heat and pressure of burial transforms the organic matter into geopolymers such as kerogen and bitumen. When the rocks containing organic matter are buried deep enough, the rocks undergo catagenesis, where temperature begins to convert the kerogen into bitumen and ultimately into hydrocarbons such as oil and gas. The rocks that produce hydrocarbons are referred to as source rocks.
Kerogen and bitumen are large organic molecules of no fixed structure. The composition of the kerogen and bitumen depends both on the type of organic matter used to produce them and the thermal maturity of the sample. While kerogen and bitumen have different molecular structures, they are typically separated functionally; the latter is soluble in common organic solvents while the former is not. The majority of bitumen is produced later during catagenesis, though a small amount occurs from diagenesis.
Understanding kerogen and bitumen properties and content is important for estimation of thermal maturity and potential hydrocarbon production. Thermal maturity indicates how much and what type of hydrocarbon is expected to be produced from an unconventional shale reservoir or a conventional reservoir sourced by a particular or multiple source rocks. In addition to kerogen and bitumen, a third class of organic matter, pyrobitumen, may exist in more thermally mature systems. Like kerogen, pyrobitumen is also insoluble in typical organic solvents. However, while kerogen originates from the originally deposited organic matter, the pyrobitumen comes from the cracking of bitumen during catagenesis and metagenesis.
The current standard method for determining thermal maturity is programmed pyrolysis, such as the “Rock-Eval™” (Vinci Technologies) or “Source Rock Analysis” techniques. These will heat up a crushed portion of sample in an oven or ovens in a series of stages at different temperatures to pyrolyse and oxidize the sample. The “Rock-Eval™” analyser, for example, includes a flame ionization detector (FID) that measures organic compound gases released during each stage of heating while sensitive infrared detectors are used to measure the quantity of CO and CO2 generated during pyrolysis and oxidation of samples. A thermocouple monitors temperatures, which are recorded on a chart known as a pyrogram. The measured organic compound gases, CO and CO2 are plotted as a function of temperature on the pyrogram. During the first heating stage, the sample is held at an initial temperature for a period of time and the produced organic products are measured. This is referred to as the S1 peak, which relates to the hydrocarbons and bitumen in the sample. The temperature is then ramped higher at a set heating rate. A second peak, S2, corresponds to the hydrocarbons that evolve from the sample during the second programmed heating stage of pyrolysis, which result from the thermal cracking of kerogen. The associated release of carbon dioxide (CO2) and carbon monoxide (CO) during pyrolysis is measured by the IR detector. The S3 peak corresponds to the amount of CO and CO2 that is evolved from thermal cracking of the kerogen during pyrolysis. This peak is associated with the organic associated oxygen in the sample. The temperature at which the S2 peak has the highest signal intensity, and thus maximum generation of hydrocarbons from kerogen, is referred to as Tmax. Tmax relates to thermal maturity, as higher temperatures are required to crack the kerogen into hydrocarbons for more thermally mature samples. There is the potential to heat the sample up to even higher temperatures and observe the produced organic products. For example, the high temperature programmed pyrolysis can be used to measure the Spy peak, which relates to pyrobitumen.
The programmed pyrolysis methods are bulk methods; the samples need to be crushed and homogenized before measurement. Therefore, any spatial information regarding the distribution of organic matter is lost during the crushing process. They are also, practically, completely destructive with respect to the samples, as the samples cannot be used for further tests after programmed pyrolysis. Programmed pyrolysis measurements are time intensive, usually requiring about an hour per sample to perform. The results also can have issues with interference from carbonate in the sample. If the samples are carbonate rich, they typically will need to be pretreated with hydrochloric acid to prevent interference in the measurement.
Thermal maturity is often estimated using the temperature where the maximum number of hydrocarbon products are produced from kerogen. This can be unreliable, as the Tmax peaks are often quite broad, such that the exact location of the peak can vary and can be difficult to reproduce with subsequent measurements. Thermal maturity calculations from Tmax are often unreliable particularly for low organic content samples. As programmed pyrolysis methods take approximately an hour per sample, this is a time intensive method to measure thermal maturity.
Fourier Transform Infrared (FTIR) spectroscopy has been used to estimate these parameters. Analysis of the FTIR spectrum with multivariate analysis has shown good predictive value for geochemical parameters such as S1, S2, and to a lesser degree S3. Predictive ability of FTIR to date for hydrogen and oxygen indices, however, has been of poor quality. FTIR suffers the same drawback of loss of spatial resolution of the organic matter as the programmed pyrolysis, as samples need to be powdered before measurement.
Laser induced breakdown spectroscopy (LIBS) uses a laser to ablate a tiny portion of sample. The standard for LIBS uses a q-switched solid state laser that produces a rapid pulse, typically on the order of pico- to nanoseconds in duration. Optics are used to focus the energy onto a single spot on the sample. The laser ablates a small amount of sample at this spot, turning it into a high temperature plasma. The excited atoms then return to a ground state, giving off light of characteristic frequencies. The spot size vaporized by the laser can range in size from a few microns up to hundreds of microns, allowing a large range of resolution and is dependent on the optics of the system. The signal quality improves with larger spot size, but sacrifices resolution. While a small amount of sample is consumed, the amount is so small that it is considered to be negligible and the technique is considered non-destructive. The wavelength of light from the plasma can be in the 180 to 980 nm region. Detection means may comprise a spectrometer adjusted to a part of the spectral region. The resulting spectra can be analysed by multivariate data analysis to correlate the spectra to concentration of elements. The spectroscopic analysis of the optical emission in LIBS is different from analytical approaches based on mass spectrometry.
LIBS has been used as a method for mineralogy identification, making it an alternative to X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) methods for mineralogical analysis of samples. It has an advantage over XRF for mineralogical identification because it can measure all elements, whereas XRF is unable to detect light elements. LIBS does have a disadvantage in terms of quantification of heavier elements compared to XRF.
Laser Induced Pyrolysis (LIPS) methods have been used previously on geological samples. LIPS relies on mass-spectroscopy methods of detecting and analysing the products of pyrolysis instead of optical emissions spectroscopy. Further, those LIPS methods appear to be limited to just total organic carbon (TOC), and do not appear to present information on thermal maturity or kerogen versus bitumen discrimination.